Coated paperboard process for manufacturing the same and products obtained thereof

ABSTRACT

The invention relates to a coated food paperboard ( 1 ), to the manufacturing process and use of the same, and to products obtained thereof. The paperboard ( 1 ) comprises one or several fibre material layers ( 2 ), such as a three-layer structure consisting of a middle layer ( 4 ) of a mixture of sulphate pulp and CTMP and of sulphate pulp layers ( 5 ) on both sides of it, and of a polymeric coating ( 3 ) getting into contact with food, either on one side or on both sides of the paperboard. According to the invention, the coating ( 3 ) consists of an outer layer ( 6 ), the melting point of the polymer used in which is at least 230° C., and of an inner layer ( 7 ) placed against the fibre material layer ( 5 ); in this inner layer, a polymer with a melting point of at least 230° C. is blended with another adhesive polymer, the melting point of which is 130-185° C. Further, the inner layer ( 7 ) may be provided with mineral substance, such as calcium carbonate. In accordance with the invention, the paperboard ( 1 ) is manufactured by coextrusion of superimposed polymeric layers ( 6, 7 ). The paperboard ( 1 ) of the invention is suitable for liquid packaging or oven board, and products manufactured of the paperboard comprise especially oven dishes and consumer packages for food, which include the dish and which may be heated as such.

This application is the national phase under 35 U.S.C. § 371 of PCTInternational Application No. PCT/FI99/00910 which has an Internationalfiling date of Nov. 1, 1999, which designated the United States ofAmerica and was published in English.

FIELD OF THE INVENTION

The present invention relates to a coated paperboard for food,comprising one or more layers of fibre material and a heat-resistantpolymeric coating, which gets into contact with food. Further, theinvention relates to the manufacturing process and use of such apaperboard, and to a number of products obtained therefrom.

BACKGROUND OF THE INVENTION

A heat-resistant oven board is used as material for such food dishes andpackages, which have to withstand heating in a common or microwave oven.A typical heatable food dish is an oven dish, which may be used as partof a finished package for heatable food, but which may also be marketedto consumers as a separate product. The polymeric coating of an ovenboard makes the paperboard water and grease resistant, and the polymerhas to withstand the temperatures used in heating, typically at least230° C.

It has been known to use polypropylene or polyesters, such aspolyethylene terephthalate (PET) as coating for oven boards. The problemwith some of these known polymers has been insufficient heat resistance.Another problem relating to all polyesters used as coatings is the pooradhesion of the polymer to the paperboard. In order to achievesufficient adhesion, the coating has to be spread onto the paperboard inlayer thicknesses which are at least approx. 35-40 g/m². If the layer isthinner, there is a danger of it coming off, for example, due to vapourreleased from the paperboard. The use of possible binding agentsimproving the adhesion has been restricted by their poor heatresistance. Pin holes are also easily left in a thin polymeric layer.

Polyesters as such, and especially PET with a high melting point, arevery well adapted to be used as coatings for oven paperboard. Thesepolymers are compatible with food, because of their sufficientmechanical strength and heat resistance, and because their organolepticadvantageousness is kept at oven temperatures, they are heat-sealableand they may be spread onto the paperboard by extrusion. The onlyproblem is caused by layer thicknesses which indicate a high materialconsumption and high amounts of polymer contained in the waste taken todumping areas or material recycling. The need for reduction in polymerconsumption is especially emphasised as even stronger and thinner fibrebases for paperboard have been developed and as the norms concerningwaste take into account also the relative amount of polymer comparedwith fibre material included, besides the absolute amount.

SUMMARY OF THE INVENTION

The object of the present invention is to form a coated paperboard, inwhich the adhesion of the polymeric coating to the paperboard has beenimproved, while simultaneously making the coating thinner and less proneto hole formation as before. It is characteristic of the paperboard ofthe invention that it is formed of superimposed polymeric layers,comprising an outer layer in which the melting point for the polymerused is at least 230° C., and an inner layer placed against the fibrematerial layer, the inner layer including polymer with a melting pointof at least 230° C. blended with another polymer achieving adhesionbetween the coating and fibre material, the melting point of thispolymer being 130-185° C.

The outer layer of the two-layer polymeric coating of the paperboard ofthe invention contacting the food thus comprises a heat-resistantpolymer with the necessary oven properties and preferably consisting ofsome known coating polymer for oven boards that has been tested in use.According to the invention, adhesion of the outer polymeric layer to thepaperboard, i.e. to the fibre material layer below, is achieved by aninner adhesion layer in which another adhesive polymer with a lowermelting point has been blended with the heat-resistant polymer. Becauseof the heat-resistant polymer melting at or above 230° C, the innerlayer withstands heating in a stove or microwave oven at the same timeas it is modified by said polymer melting at lower temperatures so thatits adhesion to the fibre material is substantially better than before.Due to improved adhesion, the two-layer polymeric coating for thepaperboard of the invention may be produced with a considerably smalleramount of polymer, even when added together, than the previous one-layercoatings for oven paperboards.

Besides oven boards, the paperboard of the invention is also applicableto liquid packaging boards, e.g. drinking cups and other disposabledishes, due to its compatibility with food and its heat-sealability.

The mixture ratio of polymers in the inner coating layer of thepaperboard of the invention preferably is within the range of 85-97% ofpolymer with a higher melting point (≧230° C.) and 3-15% of adhesivepolymer with a lower melting point (130-185° C.). The polymer with thehigher melting point is most preferably the same polymer as has beenused in the outer layer of the coating.

A preferable polymer to be used in the outer coating layer and also asthe second polymeric component of the inner coating layer ispolyethylene terephthalate, such as Eastapak PET copolyester with amelting point of 240° C., manufactured by Eastman Chemical Company, orEastapak PET polyester 12440, manufactured by the same company. InPET-based copolyesters, comonomers used in relatively small amounts(approx. 1-10%) may comprise, for example, cyclohexane dimethanol (CHDM)or isophthalic acid (IPA), which improve the processability of thepolymer. Other polymers may comprise polybutene terephthalate,polyethylene glycol-modified polyethylene terephthalate (PETG),polyethylene naphthalene (PEN), and polypenthene, which, used as such,all adhere poorly to fibre material; but the problem of adhesion relatedwith these may be solved with a two-layer coating of the presentinvention.

The adhesive polymeric component melting at 130-185° C. for the innercoating layer may preferably comprise a terephthalate-based copolyesterwhich especially well both blends with and binds to said polyethyleneterephthalate melting at or above 230° C. An example of such polymers isEastobond copolyester 19411 manufactured by Eastman Chemical Company, inwhich the used comonomer is diethylene glycol (DEG), reducing thecrystallinity of the polymer and increasing the polarity due to its OHgroups, these factors improving the adhesion between the polymer and thepaperboard.

The invention is firer applicable in an advantageous way so that,besides the two polymeric components, the inner coating layer isprovided with fine mineral substance. It has been noted that mineralsubstance facilitates the coextrusion of polymeric layers ontopaperboard and makes it possible for the polymeric layer to be eventhinner than before. An especially suitable mineral is, for example,calcium carbonate but, for example, also titanium oxide or talcum areusable. When using a mineral component, the composition of the innercoating layer may preferably vary within the range of 80-90% ofpolymeric component melting at a higher temperature (≧230° C.), 3-10% ofadhesive polymeric component melting at a lower temperature (130-185°C.), and 5-15% of mineral substance.

The total weight of a two-layer polymeric coating of the paperboard ofthe invention is preferably at most 25 g/m², most preferably 15-22 g/m².By using said mineral substance as part of the inner layer, the totalweight of the coating may be in an even lower range, i.e. 13-22 g/m².

The fibre base for the paperboard of the invention preferably consistsof a three-layer paperboard, in which the middlemost layer is a thickerlayer of a mixture of chemical pulp and CTMP, and the layers on bothsides of this layer substantially consist of pure chemical pulp. Thepolymeric coating may be spread either on only one side or both sides ofthe paperboard.

It is characteristic of the method for manufacturing a coated paperboardof the invention that the polymer forming the outer layer of the coatingand the polymeric mixture forming the inner layer are togethercoextruded onto a moving paperboard web. Extrusion is followed by rapidcooling during which the polymer remains in an amorphic state so that itis heat-sealable. Most preferable for coextrusion is a layer combinationin which the outer layer to be placed onto the paperboard consists ofpolyethylene terephthalate, such as said PET polyester 12440 or PETcopolyester 9921, the inner layer comprising a mixture of polyethyleneterephthalate, such as either of the said products, of aterephthalate-based copolyester, such as the said Eastman copolyester19411, and of calcium carbonate. However, in the applications of theprocess, the layers to be coextruded may vary, as is apparent of theabove description of the paperboard of the invention.

Products to be manufactured of the paperboard of the invention comprise,above all, paperboard oven dishes for heating food in a common ormicrowave oven. For protecting the paperboard base of the dish and forpreventing the food from sticking to the dish, the polymeric coating ofthe paperboard has to be provided at least to the interior surface ofthe dish. However, it is preferable to provide both the interior andexterior surface of the dish with a polymeric coating, in which case thepaperboard is also protected from possible spatters and grease in thebaking plate.

The oven dish of the invention may be formed of paperboard coated withpolymer either by pressing or, alternatively, by folding and jointsealing the folds thus produced to the exterior surface of the dish. Ineither case, the polymeric coating withstands the joint sealing andother moulding measures without hole formation or breaking.

According to the invention, the oven dish may form part of a finishedpackage for food, the package and its contents being heatable as such ina stove or microwave oven. Besides the dish and the heatable food packedinto it, such a package comprises a separate protective cover orwrapping, which is removed before heating, when necessary.

As mentioned above, the paperboard of the invention is also usable inproducts manufactured of liquid packaging board, such as disposablecups, plates and packages. Here the advantage of polyesters, such asPET, is that they are heat-sealable and that they do not give any tasteor smell to foods in contact with them. With the invention, it has beenpossible to improve the mutual adhesion of PET and paperboard and toreduce the thickness of the PET layer.

The invention further comprises the use of the coated paperboarddescribed above as oven board or as liquid packaging board and,especially, the use of it as part of a consumer package shaped as adish, containing heatable food.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is next described in more detail with the help ofexamples, referring first to the enclosed drawings, in which

FIG. 1 is a schematic view of a polymer-coated paperboard of theinvention;

FIG. 2 is a schematic view of a second paperboard of the invention;

FIG. 3 shows an oven dish manufactured of paperboard of FIG. 2; and

FIG. 4 shows on enlarged scale a section of the wall structure of a dishin accordance with FIG. 3 at the edge of the dish.

DETAILED DESCRIPTION OF THE INVENTION

In FIG. 1, there is shown the layer structure of paperboard 1, the otherside of which is provided with a polymeric coating. The fibre layers ofthe paperboard are together referred to with the reference number 2 andthe polymeric coating with the reference number 3 in the figure.

The fibre layers 2 consist of a three-layer paperboard, the thickermiddle layer 4 of which is a mixture of sulphate pulp and CTMP, theouter layers 5 on the both sides of the middle layer consisting ofsulphate pulp. The share of the middle layer 4 of the weight of thefibre layers 2 is approx. 60%, and the share of both the said sulphatepulp layers 5 is approx. 20%. The total weight of the fibre layers 2without the coating layers may be approx. 200-400 g/m², for example,approx. 225 g/m². When necessary, filler (not shown) may be used on thesurfaces of the three-layer paperboard. In FIG. 1, the polymeric coating3 consists of an outer layer 6, in which a polymer compatible with foodhas been used, the melting point of which is at least 230° C., and of aninner layer 7 coextruded with the outer layer, the inner layercomprising a polymer, the melting point of which is at least 230° C.,blended with another adhesive polymer with a melting point of 130-185°C. Besides the said polymeric components, the inner layer 7 may containa fine mineral substance, such as calcium carbonate. The polymericcoating 3 may consist of an outer layer 6 of for example, polyethyleneterephthalate with a melting point of 240° C., and of an inner layer 7,which includes a mixture of 80-90% of the same polyethyleneterephthalate as in the outer layer, 3-10% of adhesiveterephthalate-based copolyester with a melting point of 159° C., and5-15% calcium carbonate. The added weight of the layers 6, 7 of thepolymeric coating 3 then is preferably 13-20 g/m².

The coated paperboard of FIG. 2 is structurally similar to that shown inFIG. 1, with the exception that its both sides are provided withpolymeric coating 3. The fibre layers 2 are thus formed of a similarthree-layer paperboard as in the application in FIG. 1, and also thematerials and weight of the polymeric coatings 3 on both sides of thepaperboard 1 may correspond to those shown in FIG. 1. The paperboard ofboth FIG. 1 and FIG. 2 is suitable to be used as material for ovendishes; in this case, the paperboard of FIG. 1 has the advantage thatthe amount of polymer in relation to fibre material is smaller, and theadvantage of the application in FIG. 2 is that the paperboard is betterprotected from moisture and grease.

In FIG. 3, there is shown an oven dish manufactured of the paperboard ofFIG. 2, which, for example, is suitable for a package for ready-preparedfoods. The dish 8 thus comprises a polymeric coating 3 both on itsinterior and exterior surface, as is shown in FIG. 4.

EXAMPLES Example 1

One side of a three-layer paperboard with a weight of 295 g/m² andpropagation speed of 300 m/min, was provided by coextrusion with a toplayer of Eastapak PET copolyester 9921 and with an inner layercomprising a mixture of 80 weight % of Eastapak PET copolyester 9921, 10weight % of adhesive Eastobond copolyester 19411, and 10 weight % ofcalcium carbonate. The extrusion temperature was 290° C. and the airgap, i.e. the distance of extrusion nozzles from the paperboard web, was18 cm. The weight of both the extruded polymeric layers was 11 g/m²,i.e. the total weight of the two-layer coating thus obtained was 22g/m².

Examples 2-4

The procedure of Example 1 was followed with the exception that theweights of the layers extruded in weight ratio 1:1 were 10 g/m², 9 g/m²and 8 g/m², and, respectively, the total weights of the two-layercoating were 20 g/m², 18 g/m² and 16 g/m².

Example 5

A two-layer coating was coextruded on both sides of a three-layerpaperboard with a weight of 295 g/m² and propagation speed of 300 m/min,the top layer of the coating comprising Eastapak PET copolyester 9921and the inner layer being a mixture of 80 weight % of Eastapak PETcopolyester 9921, 10 weight % of adhesive Eastobond copolyester 19411,and 10 weight % of calcium carbonate. The extrusion temperature was 290°C. and the air gap, i.e. the distance of extrusion nozzles from thepaperboard web, was 18 cm. The weight of each of the extruded polymericlayers was 11 g/m² so that a two-layer coating with a total weight of 22g/m² was produced on both sides of the paperboard.

Example 6

The procedure of Example 5 was followed with the exception that theweight of each of the extruded polymeric layers was 10 g/m² so that atwo-layer coating with the total weight of 20 g/m² was produced on bothsides of the paperboard.

Reference Material

One side of a three-layer paperboard with a weight of 295 g/m² andpropagation speed of 300 m/min was extruded with Eastapak PETcopolyester 9921 to produce a one-layer coating. The extrusiontemperature was 290° C. and the air gap in extrusion was 18 cm. Theweight of the extruded layer was 22 g/m².

Tearing Tests

The adhesion of the coating was evaluated from the coated paperboardsproduced in accordance with Examples 1-4 and the reference material bytearing off the polymeric coating from dry paperboard. The evaluationtook place on a scale of 1-6, in which the best value 1 represented a100% fibre tear, the surface of the polymeric coating torn off thepaperboard then being totally covered by fibres, the poorest value 6representing missing adhesion, in which case the surface of the coatingtom off from the paperboard was clear, without adhered fibres. The value6 indicates a rejected result, while the value 5 again is stillsufficient to keep the coating adhered to the paperboard as thepaperboard is being processed into a food dish.

Further, wet adhesion was evaluated from the same materials by soakingthe paperboard samples in room-temperature water and by tearing off thepolymeric coating from the completely waterlogged paperboard. The scalewas 1-3, the values 1 and 2 representing an approved and the value 3 arejected result.

The test results have been shown in the following Table 1.

TABLE I Material Dry adhesion Wet adhesion Example 1 1 1 Example 2 1 1Example 3 3 2 Example 4 4 2 Reference material 6 3

Processing into an Oven Dish and Heating Tests

Oven dishes were manufactured of paperboards with polymeric coatings inaccordance with Examples 1-4 by stamping a paperboard with a moisturerate of 11% by a hydraulic compression tool, the temperature of whichwas 200° C. The dishes thus obtained were then kept in an oven with atemperature of 230° C. for 30 minutes.

In stamping, the paperboard is placed into a high temperature for ashort time, the moisture contained in the paperboard evaporating andgenerating a pressure which may remove some of the coating. Also in anoven there is the danger of gas bubbles forming beneath the coating andthe coating unsticking. In the tests carried out, the paperboards inaccordance with Examples 1-4, however, withstood the stamping andheating in an oven without any signs of the coating unsticking or ofsome other damages.

Heat Sealing Tests

Drinking cups were manufactured of paperboards coated on both sides withpolymer according to Examples 5 and 6 by heat sealing at a sealingtemperature of 250° C. Risk factors in the joint sealing are that thejoints remain leaking or that pin holes will exist in the area of jointsin the coating layer, due to heating.

Sealability was evaluated by filling the cups with a test liquidcontaining approx. 50% of water, approx. 50% of ethanol and a smallamount of colouring agent, the penetration of which into the paperboardis an easily observable sign of failed sealing. In the test, no leakswere observed in the joints of neither cup manufactured of the cardboardof the example.

It is obvious for one skilled in the art that the applications of theinvention are not limited to the above examples, but they may varywithin the scope of the following patent claims.

We claim:
 1. A coated food paperboard comprising one or several fibermaterial layers and a heat-resistant polymeric coating getting intocontact with food, said coating consisting of superimposed polymericlayers comprising an outer layer, the melting point of the polymer ofwhich is at least 230° C., and an inner layer placed against the fibermaterial layer, to achieve adhesion between the coating and the fibermaterial, characterized in that the inner layer comprises a firstpolymer with a melting point of at least 230° C., blended with a secondpolymer which is an adhesive polymer with a melting point of 130-185° C.in a ratio of 85-97% by weight of said first polymer and 3-15% by weightof said second polymer.
 2. Paperboard according to claim 1,characterized in that the polymer of the outer layer and the one of thepolymers of the inner layer are of the same polymeric material. 3.Paperboard according to claim 2, characterized in that the outer layerof the coating is polyethylene terephthalate, and the inner layer is amixture of polyethylene terephthalate and a terephthalate-basedcopolyester with a lower melting point.
 4. Paperboard according claim 1characterized in that the total weight of the polymeric coating is atmost 25 g/m².
 5. Paperboard according to claim 4, wherein the totalweight of the polymeric coating is 15-22 g/m².
 6. Paperboard accordingto claim 1, characterized in that the inner layer of the coating furtherhas blended in it fine mineral substance.
 7. Paperboard according toclaim 6, characterized in that the total weight of the coating is atmost 25 g/m² by weight.
 8. Paperboard according to claim 7, wherein thetotal weight of the coating is 13-22 g/m².
 9. Paperboard according toclaim 1, characterized in that the inner layer comprises 80-90% byweight of polymer with a melting point of at least 230° C., 3-10% byweight of polymer with a melting point of 130-185° C., and 5-15% byweight of mineral substance.
 10. Paperboard according to claim 5 or 9,characterized in that the mineral substance is calcium carbonate. 11.Paperboard according to claim 10, characterized in that the outer layerof the coating is polyethylene terephthalate and the inner layer is amixture of polyethylene terephthalate, a terephthalate-based copolymerwith a lower melting point, and calcium carbonate.
 12. Paperboardaccording to claim 1, characterized in that the fiber material layerscomprise a three-layer structure, in which the middlemost layer is athicker layer consisting of a mixture of chemical pulp and CTMP, thethinner layers on both sides of it consisting essentially of purechemical pulp.
 13. A method for manufacturing a coated paperboardaccording to claim 1, characterized in that the polymer forming theouter layer of the coating and the polymeric mixture forming the innerlayer are coextruded together onto a moving paperboard web.
 14. Aheat-resistant oven board comprising the coated food paperboard ofclaim
 1. 15. A consumer package shaped as a dish for heatable foodcomprising the coated food paperboard of claim
 1. 16. A liquid packagingboard comprising the coated food paperboard of claim
 1. 17. An ovendish, characterized in that it has been manufactured of the paperboardaccording to claim 1 so that the polymeric coating of the paperboard isattached to the interior surface of the dish.
 18. An oven dish accordingto claim 17, characterized in that it has been manufactured ofpaperboard by compression.
 19. An oven dish according to claim 17,characterized in that it has been manufactured of paperboard by foldingand joint sealing the folds thus produced to the exterior surface of thedish.
 20. A heatable food package, characterized in that it comprisesthe oven dish according to one of the claims 17-19, food intended to beheated in the dish, and a removable protective cover or wrapping closingthe dish.
 21. A coated food paperboard comprising one or more fibermaterial layers and a heat-resistant polymeric coating for contactingfood, said heat-resistant polymeric coating comprising superimposedpolymeric layers comprising an outer polymeric layer for contactingfood, wherein the melting point of the polymer of the outer polymericlayer is at least 230° C., and an inner adhesive layer in direct contactwith the outer layer and placed against the fiber material layer inorder to achieve adhesion between the coating and the fiber material,characterized in that the inner layer comprises a first polymer with amelting point of at least 230° C., blended with a second polymer whichis an adhesive polymer with a melting point of 130-185° C. in a ratio of85-97% by weight of said first polymer and 3-15% by weight of saidsecond polymer.